Filter materials for producing interchangeable filter elements in hydraulic systems are known in diverse configurations and include, for example, of a filter nonwoven, preferably of several layers, with a support structure on one or both sides (incident flow side or outflow side). A hydraulic fluid to be filtered flows through these types of filter elements, in part with a considerable pressure difference arising from the unfiltered side to the filtered side. To be able to withstand this pressure difference and dynamic flow forces in the unfiltered material, the filter materials from which corresponding filter elements are produced have support structures. Such support structures are subject to strong cyclic pressure loading in the operation of a filter element and are generally formed from metal fabrics, especially fabrics made of high-grade steel wires.
EP 1 436 062 B1 discloses a filter element for fluids, in particular hydraulic fluids, comprising a filter material of the aforementioned type. The filter material comprises a filter medium with a latticed support structure supporting the filter medium with respect to the flow direction through the filter element at least on the filtered side. The support structure is produced from a plastic material, has electrically conductive elements, is made as a support fabric of metal filaments and plastic threads and rests flat on the filter medium. The filter medium and the flat-resting support fabric or the support structure are folded up into a star shape.
Another generic filter material for fluids and a method are disclosed in DE 200 13 839 U1. The known filter material is a textile composite partially of an electrically conductive material and contains at least one support layer formed by threads arranged crosswise to one another and at least one fiber support formed by a pinned non-woven fabric. The support layer or the support structure is made as a woven fabric or a non-crimp fabric, with some of the threads of the woven fabric being formed from an electrically conductive material, preferably metal. The other threads are formed from a plastic material in the same manner as the non-woven fabric forming the filter medium. To carry out the needling process, the puncture depth is advantageously chosen such that the needles completely pierce the non-woven fabric. The puncture density is typically 800 punctures per cm2. The joining of the two layers by needling is complex and consequently costly.